In a large saucepan over medium, melt 1 tablespoon coconut oil. Add half the onion and cook, stirring, until softened but not browned, 4 to 6 minutes. Stir in 1 teaspoon ginger, the garlic and turmeric; cook, stirring, until fragrant, about 1 minute.

Stir in the lentils, 1¼ cups water, 1¼ cups coconut milk and ¾ teaspoon salt. Bring to a simmer over medium-high, then reduce to medium-low and gently simmer, uncovered and stirring occasionally, until the lentils are thick, soft and creamy, about 15 minutes. Stir in the remaining coconut milk and bring to a simmer over medium. Stir in the spinach by the handful. Reduce to low to keep warm while you make the tarka.

In a small saucepan over medium, melt the remaining 2 tablespoons coconut oil. Add the mustard seeds; cook, swirling, until sizzling and fragrant, about 1½ minutes. Add the remaining onion, ¼ teaspoon black pepper and the curry leaves (if using); cook, stirring, until the onion is lightly browned, 5 to 6 minutes. Add the remaining 2 teaspoons ginger; cook, stirring, until fragrant, about 30 seconds. Add the tomato, chili and pepper flakes; cook, stirring, until the tomato has softened, about 1 minute. Remove from the heat.

Taste and season the dal with salt and black pepper. Transfer to a serving dish. Remove and discard the curry leaves (if used) from the tarka. To serve, spoon the tarka over the dal.